Textbook Problem

After four years of experiencing amenorrhea, Tonya seeks medical care to help her become pregnant. She is convinced that her lack of menstrual periods is the cause of her infertility. Tonya’s height is 5 ft 5 in.; her weight is 107 pounds, which she has maintained for four years (she previously weighed 121 pounds). Her FSH and LH levels are both abnormally low, and she is not ovulating. When the importance and methods of weight gain are explained to her, Tonya agrees to gain some weight. After she regains 7 pounds, her LH level is normal, but her LSH level is still low, and the luteal phase of her cycles is abnormally short. When her weight reaches 119 pounds, Tonya’s LH and FSH levels, ovulation, and menstrual cycles are normal.

Questions

1.    Was Tonya underweight or normal weight based on her body mass index (BMI, kg/m²) when she weighed 107 pounds?

Summary Introduction

To explain: Whether Person T was underweight or normal weight based on her body mass index when she weighed 107 pounds.

Introduction: Body mass index (BMI) is obtained by dividing weight (in kg) by square of height (in m). If the BMI is greater than 18.5, then it is considered underweight. The BMI of 18.5-24.9 comes under normal weight, and people with 25-29.9 are overweight, and people with BMI of 30 and higher have obesity.

Explanation of Solution

Person T is 5 ft 5 in. (1.65 m); her current weight is 107 pounds (48.53 kg) and her previous weight was 121 pounds (54.88 kg). Both her FSH and LH levels are low, and she cannot ovulate. After sometime, she regains 7 pounds and her LH level becomes normal, but her FSH level is still low and the luteal phase of her cycles is abnormally short. When her weight reaches 119 pounds (53.97 kg), her LH and FSH levels, ovulation, and menstrual cycles becomes normal.

The critical level of body fat (usually indicated by a body mass index over 20 kg/m²) is needed to trigger and sustain normal reproductive functions in women. Low levels of body fat during adolescence is related to delays in the onset of menstruation and reduced fertility later in life. Impaired fertility in underweight women often takes the form of delayed time to conception and amenorrhea. Lowered libido and reduced sperm production have been identified in underweight men with low levels of body fat.

The formula for the body mass index is as follows:

$\text{Body mass index }\left(\text{BMI}\right)=\frac{\text{Weight in }\left(\text{kg}\right)}{{\left[\text{height in (m)}\right]}^2}$

Substitute $48.53\text{ kg}$ for weight and $1.65\text{ m}$ for height in the above formula.

$\begin{array}{c}\text{Body mass index }\left(\text{BMI}\right)=\frac{48.53\text{ kg}}{{\left(1.65\text{ m}\right)}^2}\\ =17.83{\text{ kg/m}}^2\end{array}$

Person T was underweight when she weighed 107 pounds as her BMI was 17.83.